Solar cells generally include two metallic electrodes for extracting electrical power generated in response to incident light. In most solar cells, at least one of the metallic electrodes is disposed on the front surface, i.e., the light-receiving surface. When such cells are connected in an array, it therefore becomes necessary for front-to-back connections between the electrodes disposed on the front and rear surfaces of the cells. In extraterrestrial applications, metallic electrodes and interconnections are susceptible to damage from plasmas, particularly from ozone plasmas. Accordingly, it is desirable to provide both cell electrodes on the rear, i.e., non-exposed, surface of the solar cells. Solar cell interconnections can thereby be protected from damage caused by ions and plasmas in space environments.
While most solar cells, particularly of the GaAs type, have front and rear electrodes and are thus subjected to the problems noted above, FIG. 11 shows a cross-sectional view of one known solar cell having both its electrodes on the rear surface of the cell. That cell construction is described in a paper by Matsunami presented at the First International PVSEC and published in the Technical Digest of that meeting, Nov. 1984, pages 133-138. The solar cell of FIG. 11 includes a p-type substrate 20 having an n.sup.30 -type diffusion layer 21 formed along the front and side surfaces of substrate 20. A p.sup.30 -type layer 22 is formed in the rear surface of substrate 20 for assistance in forming an ohmic contact with a metallic electrode 23 disposed on layer 22. The other electrode of the solar cell comprises digitated metallic stripes 24 on the front surface of layer 21 and metallic interconnections extending from the stripes along the opposite sides of the structure and terminating on the rear side. Thus, both of the electrodes of the solar cell of FIG. 11 are accessible at the rear surface of the cell.
The solar cell of FIG. 11 is fabricated in a series of processing steps beginning with coating substrate 20 with a diffusion mask, such as silicon dioxide (SiO.sub.2). A window is opened in the mask at the rear surface of the substrate and a p-type dopant is diffused through the window to form layer 22. Thereafter, the entire surface of substrate 20 is again coated with a diffusion mask. The mask is removed in the areas in which n.sup.30 -type layer 21 is to be formed, and an n-type dopant is diffused through those windows to form layer 21. These diffusions form a rectifying pn junction between substrate 20 and layer 21. Metallic electrodes 23 and 24 are deposited by conventional evaporation methods.
The solar cell of FIG. 11 does not have a particularly high light-to-electricity conversion efficiency due, in part, to the diffusion technology it employs for forming the pn junction. Furthermore, because of the requirement for forming n.sup.30 -type diffusion layer 21 on the sides of the device, each cell must be individually produced from a separate substrate. As a result, the cost of manufacturing the cells of FIG. 11 increases as the volume of cells produced increases.